Liquid crystal display devices use a backlight unit for radiating light from behind the liquid crystal display panel to illuminate the liquid crystal display panel. A backlight unit is configured using a light guide plate for diffusing light emitted by an illumination light source to irradiate the liquid crystal display panel and parts such as a prism sheet and a diffusion sheet for uniforming the light emitted from the light guide plate.
Currently, large liquid crystal televisions predominantly use a so-called direct illumination type backlight unit comprising a light guide plate disposed immediately above the illumination light source. This type of backlight unit comprises a plurality of cold cathode tubes serving as a light source provided behind the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to secure uniform light amount distribution and necessary brightness.
To achieve a uniform light amount distribution with a direct illumination type backlight unit, however, a thickness of about 30 mm in a direction perpendicular to the liquid crystal display panel is required. While demands of still thinner backlight units are expected to grow in the future, achieving a thickness of 10 mm or less with a direct illumination type backlight unit is deemed difficult in view of uneven light amount distribution expected to accompany that type.
Thus, there has been proposed a backlight unit of a type using a light guide plate that is formed of a transparent resin containing scattering particles for diffusing light (see, for example, JP 07-36037 A (PATENT DOCUMENT 1), JP 08-248233 A (PATENT DOCUMENT 2), JP 08-271739 A (PATENT DOCUMENT 3), and JP 11-153963 A (PATENT DOCUMENT 4).
For example, JP 08-271739 A discloses a planar light source composed of a wedge-shaped light diffusion light guide member formed of PMMA containing evenly dispersed therein silicone-based resin powder of small particle diameters having a different refractive index, a light source (fluorescent lamp) disposed adjacent the light diffusion light guide member, a light emission direction correcting element facing a light extraction plane of the light diffusion light guide member and having a repeated undulate pattern of prism arrays, and a reflector disposed opposite to the rear side of the light diffusion light guide member.
In the planar lighting device mentioned above, light emitted by the light source and launched or admitted through the light entrance plane into the light diffusion light guide member receives a single or a multiple scattering effect at a given rate as the light propagates through the inside of the light diffusion light guide member. Moreover, a significant proportion of light that reaches both end planes of the light diffusion light guide member or a surface of the reflector is reflected and returned into the light diffusion light guide member.
The above composite process produces light beam that is emitted through the light exit plane highly efficiently with a directivity to travel obliquely forward as viewed from the direction of the light source. Briefly, light radiated by the light source is emitted through the light extraction plane of the light diffusion light guide member.
Thus, the prior art literature mentioned above purportedly states that a light guide plate containing scattering particles mixed therein is capable of emitting uniform light with a high light emission efficiency.
As regards the light guide plate containing scattering particles mixed therein, there have been disclosed a light guide plate in the form of a flat plate and a light guide plate composed of wedge-shaped light guide plates abutted to each other, besides the wedge-shaped light guide plate mentioned above.
[PATENT DOCUMENT 1]:JP 07-36037 A[PATENT DOCUMENT 2]:JP 08-248233 A[PATENT DOCUMENT 3]:JP 08-271739 A[PATENT DOCUMENT 4]:JP 11-153963 A